A common design issue associated with ophthalmic instruments is that significant diagnostic errors can be introduced whenever a patient's eye is not positioned within predetermined bounds. Some prior art ophthalmic instruments rely on an operator's judgment and skill to visually monitor the position of the patient's eye, and manually to place it in an "acceptable" location. An Acuitus Model 5000 available from Carl Zeiss, Inc. of Dublin, Calif. is one such manually positioned, prior art ophthalmic instrument. In using this ophthalmic instrument, an operator must judge the position of the patient's eye using a video image thereof. To do this, the operator centers the pupil of the patient's eye on a video screen; the operator infers the position of the eye from the degree of focus of the video image of the pupil. As can be readily appreciated from this, eye position is problematic because the degree of focus of the video image is subjective, and it is generally not very sensitive. Thus, some error in eye position is inevitable in such a manually positioned ophthalmic instrument because of variation in operator judgment and skill.
As is also well known to those of ordinary skill in the art, ophthalmic instruments can use eye position measurement data to help correct for diagnostic measurement errors associated with residual eye position offset errors. For example, one type of prior art ophthalmic instrument uses eye position measurement data to compensate for refractor errors caused, for example, by range offset. Range offset refers to errors in positioning the instrument in the correct position along the patient's line of sight. However, despite an ophthalmic instrument's being designed to minimize diagnostic measurement errors caused by eye position offset errors, some eye position offset error sensitivity still occurs.
As one can readily appreciate from the above, a need exists in the art for a method and apparatus to determine position of an eye.